1. Field of the Invention
This invention relates to progressive cavity pumps and, in particular, to 1) methods and apparatus for monitoring the integrity of the lubrication systems used for the connecting shaft assemblies of such pumps and 2) methods and apparatus for improving the safety of such pumps under such conditions as deadhead operation, dry run operation, and lubrication system failure.
2. Description of the Prior Art
Progressive cavity pumps (pc-pumps) are widely used in the explosives industry because of their pulsation free flow, their low product shear, and their ability to handle products with up to 40% prills. They are also used in the food industry, in the handling of sewage, and in other applications where pumping of materials having relatively high abrasiveness is needed.
As shown in FIG. 1, a pc-pump 13 generally consists of a rotor 5 turning inside a stator 4. In a typical configuration, the rotor is geometrically a large pitched helix, while the stator can be regarded as a body with a two start helix with twice the pitch of the rotor. As a result, conveying spaces (cavities) are formed between the stator and the rotor.
During pumping, these cavities are filled with product and move continuously from the inlet 10 to outlet 11. As a result of the smooth transition from one cavity to the next, the pump delivery is almost pulsation free. The conveying spaces are sealed by the interference between the rotor and the stator. The latter is usually an elastomer 14 held within a rigid shell 15, although other configurations such as an elastomerically coated rotor can be used. The volume of the cavities during their advancement stays constant. The rotor moves radially within the stator. Other configurations besides a large pitched helix rotor in a two start helix stator can be used, including, for example, an elliptically shaped rotor in a tri-lobe stator. See, for example, Netzsch Product Catalog entitled "The New NM Series--Who would have thought you could improve a NEMO.RTM. Pump?", Netzsch Mohnopumpen GMBH, Waldkraiburg, Germany, June, 1994.
Rotor 5 is driven via drive shaft 6A and connecting shaft assembly 6B. Drive shaft 6A is connected to a suitable power source such as an electric, hydraulic, pneumatic, or other type of motor 72. To accommodate the orbital movement of rotor 5, connecting shaft assembly 6B either comprises a shaft made of a flexible material, such as, a spring steel, or comprises rigid shaft 6C provided with joints 8A and 8B at its ends as shown in FIG. 1. Such joints may, for example, be gear, pin, or universal joints.
Joints 8A and 8B are provided with seals or elastomeric boots 17 to prevent pumped material, e.g., explosives, from entering the joints. In some cases, rather than using two separate boots, an elastomeric sleeve is connected between the two joints and surrounds shaft 6C. Also, in certain configurations, a single boot can be used. See, for example, Waite, U.S. Pat. No. 3,930,765. Preferably, the joints are lubricated by a liquid, such as a lubricating oil. In such a case, the seals, boots, or sleeve, in addition to keeping pumped material out of the joints, also keep the lubricant out of the pumped material.
As shown in FIG. 1, drive shaft 6A is used to couple connecting shaft assembly 6B to the drive motor. If desired, connecting shaft assembly 6B can be connected directly to the output shaft of the motor. Also, multiple intermediate drive shafts can be used between the motor and the connecting shaft assembly. As used herein, the term "connecting shaft assembly" means the apparatus connected to the rotor (including any fixed extensions of the rotor which are considered part of the rotor), which apparatus allows the rotor to undergo orbital movement.
When pc-pumps work with explosives, they have to be guarded against excessive heat generation. During normal operation, pumped material carries heat away from the pc-pump, thus preventing the generation of excessive heat. Excessive heat, however, can be generated in cases of (1) deadhead operation and (2) dry pumping.
Deadhead operation (also known as deadhead pumping) occurs when flow from the pump is blocked. This can occur at the pump's outlet or downstream from the outlet. Deadhead pumping is potentially the most dangerous condition that can exist during the pumping of explosives. Assuming the drive motor does not stalls the total drive energy supplied to the pump is converted into heat, which is absorbed by the trapped explosives and by the rotor and the stator.
The rate of temperature rise depends on power input, heat sink capacity and heat dissipation of the system. When the decomposition temperature of the explosives is reached (e.g., a temperature above about 200.degree. C. for emulsions), the entire plug of explosives within the pc-pump deflagrates, which generally results in pump destruction, physical damage to the surroundings, and serious injury to personal who may be in the vicinity of the pump.
Moreover, such a primary event may lead to secondary events if fragments from the pump provide sufficient shock impetus to detonate explosives in the vicinity of the pump. As a result of these considerations, deadhead pumping incidents are a serious concern to the explosives industry and much effort has been expended to try to reduce the probability of their occurrence.
Dry pumping occurs when a pc-pump is turning but no product is available on the suction side of the stator. When a pump runs in such a dry condition, it gains heat from friction and from work derived from the deformation of the elastomer of the stator. Since no product is available to carry the heat away, it has to be absorbed by the rotor, stator, and the thin film of explosives residue which remains within the stator. As the temperature increases, the stator expands mostly inwards because of its confining rigid outer shell. This, in turn, accelerates the heating and may result in ignition of the explosives residue in the pump.
Dry pumping is generally a lesser problem than deadhead pumping because there is less explosives in the pump, but the danger is still significant. Also, dry pumping tends to occur more often. For example, operators in dealing with an air-locked pump have been known to try to solve the problem by simply continuing to run the pump, rather than taking the time to prime the pump. Operators have also been known to disable conventional safety mechanisms to allow such unsafe procedures to be used. This unfortunate fact of life is one of the reasons that safety systems which are difficult to override are needed. As discussed below, the present invention provides such safety systems.
A third dangerous condition may occur when explosives enter the joints at the ends of the connecting shaft assembly as a result of a break in the integrity of the boot, seal, or sleeve which surrounds those joints. Although the sliding velocities in such joints are low, the contact pressure between the metallic parts is high and this can lead to increased friction especially when the lubricant is lost and replaced by explosives. Explosives are always sensitive to friction and can become even more so through crystallization and water loss. The friction levels in a joint can thus be high enough to ignite explosives. This constitutes a hazard.
When non-explosive materials are being pumped, the danger of an explosion, of course, does not exist. However, presence of pumped material in the joints is not desirable since it shortens the life of the pump and can lead to contamination of the pumped material by, for example, metal particles and the lubricant.
Numerous approaches have been used in the prior art to address the foregoing problems. These approaches have usually been electronic in nature and have sensed no flow, high and/or low pressure, or high temperature, all of which are indicators of unsafe conditions. Devices embodying these approaches have generally been sensitive and relatively delicate. Accordingly, they have worked well in a controlled environment, but have been less fail proof in a rough environment, such as on explosives pump trucks or underground explosives loading equipment. Another drawback is that these devices have generally been too easy to by-pass.
Examples of the prior art approaches include thermal dispersion flow sensors, Coriolis (U-tube) flow meters, pressure differential flow meters; devices for detecting absolute pressure levels, devices for monitoring supply levels of explosives to avoid dry pumping, pressure relief valves, thermofuses, bursting discs, and shut-off timers which must be reset before further pumping is permitted. Many of these devices are used in feedback loops to interrupt the supply of electrical or hydraulic power to the drive motor for the pump. See ICI Explosive Pump Code, ICI International Inc., London, England, Jun. 16, 1992, pages 13-16 and 37-46.
Along these lines, efforts have been made to measure the temperature between the rotor and the stator of a pc-pump using a thermistor sensor, and to then use the output of the sensor to control the operation of the pump's motor. See Pumpen-Und Maschinenbau product brochure entitled "SEEPEX.RTM. Dry Running Protection TSE," Pumpen-Und Maschinenbau Fritz Seebergerkg, Bottrop, Germany, Publication No. 700.
Also, efforts have been made to reduce the damage caused by a deflagrating pump, e.g., by using a stator which bursts at a preset internal pressure. See, for example, U.S. Pat. No. 5,318,416.
As discussed fully below, the present invention significantly improves on these prior safety approaches for pc-pumps. If desired, the present invention can be used in combination with one or more of these prior approaches, e.g., in combination with bursting discs or a stator which bursts at a preset internal pressure.
The integrity of boots 17 used to isolate joints 8A and 8B of connecting shaft assembly 6B has been tested in the past by 1) forming channels within drive shaft 6A and connecting shaft 6C and 2) equipping the drift shaft with a fitting for applying pressure to the drive shaft channel. The channels in the drive shaft and the connecting shaft communicated with the boots and thus boot integrity could be checked by applying pressurized air to the fitting and detecting the decline in pressure (if any) over time. This system suffered from a number of problems, including the fact that detection of boot integrity was not performed continuously and the fact that explosives entering a joint through a ruptured boot could block a channel so that the pressure test would indicate an intact boot, when in fact the boot was ruptured. See ICI Explosive Pump Code, ICI International Inc., London, England, Jun. 16, 1992, pages 18-19 and 57.
Examples from the patent literature of approaches which have been proposed to improve the safety of pc-pumps include Byram, U.S. Pat. No. 2,512,765, Hill, U.S. Pat. No. 2,778,313, and Marz, EPO Patent Publication No. 255,336.